The present invention provides a granular product of parahydroxybenzoic acid or a parahydroxybenzoic acid ester having significantly suppressed dusting and caking tendency, and a process for preparing the same.
Parahydroxybenzoic acid can be employed as a monomer component for a wide variety of polymer products. Recently, it draws the attention of the art as a monomer component for preparing liquid crystalline polymers, which exhibit high strength and high elastic modulus. In addition, many of its alkyl esters have been employed as fungicide for cosmetic products or industrial purposes. Generally, parahydroxybenzoic acid is prepared by a method comprising the steps of reacting phenol with potassium hydroxide to give potassium phenoxide, reacting the obtained potassium phenoxide with carbon dioxide under pressure to give potassium parahydroxybenzoate and then, isolating the desired compound by means of acid precipitation, i.e. by adding a mineral acid to the salt.
For a long time, the Kolbe-Schmitt reaction, a solid-gas phase reaction, had been employed for the reaction of potassium phenoxide and carbon dioxide. Said reaction, however, has some problems such as long reaction time due to the solid-gas phase reaction, high amount waste of starting materials in side reaction because of thermal heterogeneity, difficulties in controlling the reaction which makes it difficult to attain a stable yield. In order to solve those problems, a number of methods had been proposed.
One of the instant inventors had proposed a process, which can produce the compound in a short time with high yield in a continuous manner. Said process comprises the step of reacting potassium phenoxide and carbon dioxide in the presence of an appropriate solvent at a temperature equal to or more than 180xc2x0 C. in a dispersion, provided that phenol in an amount required to react with dipotassium parahydroxybenzoate to give potassium phenoxide is added to the reaction before the initiation of the carboxylation step (Japanese Patent Publication (KOKOKU) No. 9529/1970).
Thus obtained parahydroxybenzoic acid is then isolated from the solution by means of, such as, aciding out, filtration or centrifugation, and washed to give the compound to be used as a monomer component of polymers such as liquid crystalline polymers.
Parahydroxybenzoic acid esters may be prepared by a conventional process to obtain carboxylate esters; for example, by means of dehydration reaction between parahydroxybenzoic acid and an alcohol under the presence of acid catalyst such as sulfuric acid.
Crystalline product of parahydroxybenzoic acid or its ester usually comprises very fine particles and, therefore, is highly dusty causing operation problems. For example, when parahydroxybenzoic acid or its ester is added into a reaction vessel as a monomer component of a polymer or as a fungicide, fine particles of the compound fly in the air as powdery dust. The dust of parahydroxybenzoic acid or its ester flown in the air is hardly precipitated, and the widely dispersed dust causes problems in handling the same.
In order to diminish the problems concerning workability and safety in the feeding step, operators wear dust-proof glasses and masks and the reactor is mounted a vacuum at a position other than the supply port and a filter to trap the fine particles. However, they are not enough.
Upon storage, the fine particles of parahydroxybenzoic acid or its ester tend to form cake and the caking tendency also causes problems in handling the same. Since the specific surface area increase as the diameter of the fine particles decrease and parahydroxybenzoic acid or its ester is soluble to water, the fine particles apt to bridge each other due to moisture in the air to form cakes and the caking tendency may be augmented by the capillary phenomenon.
Further, the fine particles of parahydroxybenzoic acid or its ester are easily to be charged and therefore, adhere to the surfaces of containers or plastic bags because of the electrostatic action.
In order to suppress dust of the material with the above-described characteristics, granular product prepared by a wet extrusion granulating process comprising steps of adding water to the particles of parahydroxybenzoic acid or its ester, kneading and granulating the same by means of extrusion granulator and drying could be proposed. Although thus obtained granules exhibit somewhat suppressed dusting and electrostatic properties, the particles agglomerate more easily due to the added water and therefore, the problem of caking still remains. In addition, thus obtained granules are made of small particles which are attracted each other with relatively low bonding force, and tends to be degraded during transportation to give the original dusty small particles.
On the other hand, in the field of pharmaceutical preparation, it has been proposed that good granular product may be obtained by a method comprising the steps of adding binder component such as dextrin, starch and carboxymethyl cellulose as well as water or alcohol to the base component, kneading and granulating the same. However, adding a binder component during preparation of parahydroxybenzoic acid will deteriorate the purity of the product and polymer products or molded articles made from such a product might have imposed color tone or deteriorated properties.
Accordingly, the object of the present invention is to solve the above-mentioned problems and to provide granular product of parahydroxybenzoic acid or its ester with well-suppressed dusting and caking tendency.
The present invention provides a granular product of parahydroxybenzoic acid or its ester having an average particle size of equal to or more than 150 xcexcm and a hardness of 10-3000 g.
The granular product of the present invention exhibits well suppressed dusting tendency and therefore, is easy to handle and affects little to environment and human beings. In addition the product exhibit suppressed caking tendency which also causes improved workability of the product. In addition, the granular product of parahydroxybenzoic acid or its ester is strong enough to resist against fairly strong impact such that it is not easily degraded to the original fine particles and therefore degradation of the product during transportation or the like is well suppressed.
The granular product of the present invention may be prepared by a method comprising the steps of, dry compressing powdery parahydroxybenzoic acid or its ester to give a compressed material, pulverizing and classifying the same. Accordingly, the method for preparing the granular product is also in the scope of the present invention.
Average Particle Size
An average particle size represents the value measured as follows:
A sample product is weighted and then is sequentially screened with sieves having aperture of 2800 xcexcm, 1700 xcexcm, 1180 xcexcm, 840 xcexcm, 500 xcexcm and 250 xcexcm in this order. Firstly, the weighted sample product is screened with the sieve having aperture of 2800 xcexcm at 230 rpm for 10 minutes. The amount of the residues on the sieve was weighted and the weight ratio (wt %) to the starting amount is calculated. The sample passed through the 2800 xcexcm sieve was then screened with the sieve having aperture of 1700 xcexcm in the same manner as above. These steps repeated successively and at the last, the amount of the product passed through the 250 xcexcm sieve is weighed. The average particle size is the value calculated as follows: average particle size (xcexcm)=(2800xc3x97residue on the 2800 xcexcm sieve (wt %)/100)+(1700xc3x97residue on the 1700 xcexcm sieve (wt %)/100)+(1180xc3x97residue on the 1180 xcexcm sieve (wt %)/100)+(840xc3x97residue on the 840 xcexcm sieve (wt %)/100)+(500xc3x97residue on the 500 xcexcm sieve (wt %)/100)+(250xc3x97residue on the 250 xcexcm sieve (wt %)/100)+(120xc3x97passed the 250 xcexcm sieve (wt %)/100).
Hardness
Hardness is measured by the simplified granular hardness meter. The granular sample is applied load by means of a conical push-bar with 1 mmxcfx86 head and the weight at where the sample is disintegrated is taken for hardness of the sample.
Degradation Ratio Test Measured By Defacement Tester.
The degradation ratio test determines degradability of a granular product. Ten grams of the sample product is sieved with 60M-mesh screen (sieve having aperture of 0.25 mm) at 230 rpm for 1 minute. The residue on the sieve is loaded into defacement tester having inner diameter of 27 cm and thickness of 4 cm and is subjected to impact stress at 25 rpm for 3 minutes. Thus treated sample is sieved again with the 60M-mesh screen for 1 minute. The degradation ratio (%) is calculated with the amount of residue on the mesh before loading impact stress (W1) and those after loading impact stress (W2) according to the following formula:
Degradation ratio (%)=(W1xe2x88x92W2)/W1xc3x97100 
Angle of repose, aerated bulk density and packed bulk density are determined by means of Powder Tester (Type PT-N), Hosokawa micron Co., according to the manufacture""s instruction.
Angle of Repose
Sample is shaken on the standard sieve (10 mesh) to allow falling through a funnel and the angle of repose is measured by means of the pouring method.
Aerated Bulk Density
Sample is shaken on the sieve to allow falling into a standard container through the shout, and then the standard container is weighted to determine the aerated bulk density.
Packed Bulk Density
The sample is filled into a standard container, the container is tapped from a given height for given times and then, bulk density of the sample packed by tapping impact is determined.
Compression Ratio
The compression ratio is the value obtained according to the following formula:
xe2x80x83(packed bulk densityxe2x88x92aerated bulk density)/packed bulk densityxc3x97100
According to the present invention, the average particle size of the granular product of parahydroxybenzoic acid or its ester is equal to or more than 150 xcexcm, preferably, 250-3000 xcexcm, more preferably, 350-1600 xcexcm. In case of the average particle size is less than 150 xcexcm, satisfied dusting and caking suppression may not be achieved. In case of the average particle size is over 3000 xcexcm, the product may exhibit well-suppressed dusting as well as caking tendency, but also low dissolution rate which may cause operation problems.
According to the present invention, the granular product of parahydroxybenzoic acid or its ester may contain less than 15 wt %, more preferably, less than 6 wt % of particles that pass through the 74 xcexcm sieve. When the composition contains more than 15 wt % of the small particles that pass the 74 xcexcm sieve, the product will dust extensively due to those small size particles.
According to the present invention, the hardness of the granular product of parahydroxybenzoic acid or its ester is 10-3000 g, and preferably, 10-1000 g. When the hardness is less than 10 g, the granular product may be easily degraded during transportation or the like into fine particles, which cause dusting. When the hardness is over 3000 g, the bonding force among the particles constituting the granule is too strong to be dissolved in a medium and may cause operation problems.
The granular product of parahydroxybenzoic acid or its ester may preferably be enough strong against impact or vibration stress such that it does not degrade easily into fine particles upon certain impact. Degradation ratio of the product measured by defacement tester, which is the index representing impact strength of the product, may be equal to or less than 3% such that the product is easy to handle in transporting and operating.
In addition, the angle of repose of the granular product of parahydroxybenzoic acid or its ester may be 30-50xc2x0, preferably, 35-45xc2x0. This range of the angle of repose demonstrates improved fluidity and therefore, improved handling property due to the larger particle size of the product.
The aerated bulk density of the granular product of parahydroxybenzoic acid or its ester may be 0.5-0.85 g/cc, preferably, 0.55-0.8 g/cc. The packed bulk density may be 0.55-0.9 g/cc and preferably, 0.6-0.85 g/cc.
Further, the compression ratio of the granular product of the present invention, which is the value obtained according to the following formula:
xe2x80x83(packed bulk densityxe2x88x92aerated bulk density)/packed bulk densityxc3x97100
may be equal to or less than 10%, preferably, equal to or less than 7%.
The compression ratio of the conventionally available powdery parahydroxybenzoic acid or its ester having a small particle size is as high as 30-60%; i.e. difference between the aerated bulk density and the packed bulk density is significant.
Accordingly, if the conventional powdery product of parahydroxybenzoic acid or its ester is packed in a standard container, there will be significant amount of air spaces between the particles.
To the contrary, the low compression ratio, as low as less than 10%, of the granular product of the present invention means the improved filling property, that is, the product filled even without tapping vibration or impact contains only small amount of air spaces.
According to the present invention, the granular product of parahydroxybenzoic acid or its ester may preferably be charged equal to or less than 0.02 xcexcC/g. Due to the small amount of electrostatic charge, the problem of adhesion the product to the surfaces of containers or plastic-bags with electrostatic force is suppressed and therefore, the workability of the product can be substantially improved.
The granular product of parahydroxybenzoic acid or its ester of the present invention may be prepared by dry compressing powdery parahydroxybenzoic acid or its ester, pulverizing the compressed material and classifying the pulverized material. The starting material, parahydroxybenzoic acid or its ester may be any of those obtained by a conventional method. For example, alkaline metal salt of parahydroxybenzoic acid may be prepared according to the Kolbe-Schmidt reaction disclosed in Japanese Patent Publication (KOKOKU) No. 9529/1970, and then, parahydroxybenzoic acid may be obtained by aciding out the alkaline metal salt of parahydroxybenzoic acid at 80-100xc2x0 C., and if desired, purifying. In such a method, the acid used in the aciding out step is not particularly limited, and an inorganic or organic acid may be used. Examples of inorganic acids include binary acid (hydrogen acid) such as hydrochloric acid and hydrofluoric acid, oxo acids such as nitric acid, sulfuric acid, phosphoric acid and perchloric acid. Examples of organic acids include fromic acid, acetic acid and phenol. The pH of the reaction in the aciding out step may be adjusted to 1-4 by means of the acid as above.
Parahydroxybenzoic acid esters may be prepared by a conventional method for preparing carboxylate esters. For example, the ester can be prepared by dehydration reaction between parahydroxybenzoic acid and an alcohol under the presence of an acid catalyst such as sulfuric acid.
Examples of parahydroxybenzoic acid esters preferably granulated according to the instant invention include alkyl esters having 1-4 carbon atoms such as methyl, ethyl, propyl, isopropyl, butyl and isobutyl ester and benzyl ester.
According to the present invention, thus obtained powdery parahydroxybenzoic acid or its ester is dry compressed, the compressed material is then pulverized and classified to give the granular product of the present invention.
Generally, the term xe2x80x9cgranulatingxe2x80x9d or xe2x80x9cgranulationxe2x80x9d represents a process to provide granular product consisting of particles each having almost same size and shape, from a starting material in the form of powder, melt or aqueous solution. There are many known granulation processes such as extrusion, spray drying, milling, mixing and fluidized bed granulation. In the process of the present invention, the powdery parahydroxybenzoic acid or its ester is dry compressed with a compressor to give compressed material, the compressed material is then pulverized and classified to give granular product of the present invention. This type of procedure is generally called as dry milling granulation.
The dry compressing step may be carried out mechanically under compression pressure of 0.2-2.0 ton/cm. When the compression pressure is less than 0.2 ton/cm, the bonding force among the fine particles constituting the resulting granule becomes weak and the granule may easily be degraded. When the pressure is over 2.0 ton/cm, the bonding force becomes too strong and the solubility of the granule, which can be used as fungicide for cosmetic products or industrial purposes, to alcohol or the like is spoiled. The roll used in the dry compressing step may preferably be corrugated roll or smooth slit roll.
The compressed material obtained by mechanical compression is then pulverized by means of a mill and then classified to give granular product of parahydroxybenzoic acid or its ester with certain particle characteristics. The mill used in this step may be, for example, roll mill, medium mill, gas flow mill, shearing and grinding mill, and the like, and hammer type high speed rotary impact mill is preferably used.
The pulverized granules are then classified. The classifying step may generally be carried out according to a known process, for example, by means of a mesh screen of suitable size. The smaller particles removed by the classifying step may be returned to the compressing step, and the larger particles may be returned to the pulverizing step. Accordingly, the present process can prepare granular parahydroxybenzoic acid and its ester with an excellent yield.
According to the present invention, due to the high pressure in the granulating process, the primary particles constituting the granule firmly agglomerate such that the particles are attracted by van der Waals force and electrostatic forces each other. This situation is different from those obtained by conventional wet extrusion granulation wherein the primary particles constituting the granule are associated due to surface tension of the binder component. Therefore, the granular product of parahydroxybenzoic acid or its ester obtained according to the present invention is strong enough to resist against impact stress and can be dissolved in a medium quite easily, and therefore, is excellent in operability.
According to the present invention, the parahydroxybenzoic acid or its ester having a small average particle size used as starting material may be any of those prepared by a conventional method and employed without adjusting water content. Accordingly, the step adding water to the starting powdery material, which was essential for the conventional wet extrusion granulation process, is no longer required and therefore, the process of the present invention is suitable for large scale production.
In the present specification, the term xe2x80x9cdry compressingxe2x80x9d represents the step to compress the material without adding any binder component such as water, and does not mean that the starting material does not contain water at all. It is preferable that water content of the starting material, powdery parahydroxybenzoic acid or its ester is less than 20%, more preferably less than 12%, especially less than 6%. By dry compressing of powdery parahydroxybenzoic acid or its ester containing only small amount of water, granular product with high impact strength and good solubility described as above can be obtained.
The granular product of parahydroxybenzoic acid or its ester of the present invention may be employed as a monomer for preparing polymer materials or as a fungicide for cosmetic products or industrial purposes.
The present invention is further illustrated by means of the attached examples.